In the adult, blood cell production or haemopoiesis takes place mainly in the bone marrow. The blood cell types produced are a reflection of the needs of the organism at any moment, for example bacterial infection leads to a large increase in neutrophil production. The rate and scale of blood cell production in vivo are regulated, at least in part, by the synthesis and release of specific cytokines both within the bone marrow and also from other tissues. Here we detail the range of cytokines which act directly on haemopoietic stem cells and myeloid progenitor cells. Also cellular systems which will permit the elucidation of the specific interactions between these various cytokines which regulate stem cell self-renewal, differentiation and proliferation are described.

The continuous production of mature blood cells is required for a wide variety of functions and is essential for survival. The physiology of haemopoiesis has been elucidated in part, and some of the complex regulatory networks which control blood cell development are now known. For example, the proliferation and development of multipotent and lineage-restricted myeloid progenitor cells are controlled by a series of haemopoietic growth factors which have recently been purified and molecularly cloned. The availability of sufficient quantities of these recombinant growth factors has enabled an elucidation of the specific target cells for these agents (Table 1). This in turn has encouraged studies aimed at identifying the intracellular mechanisms, stimulated by these growth factors, that facilitate the survival, proliferation and development of haemopoietic cells. Recent progress in these fields is considered here.

Table 1.

Growth factors which stimulate myeloid cell development

Growth factors which stimulate myeloid cell development
Growth factors which stimulate myeloid cell development

Haemopoiesis

All mature blood cells: erythrocytes, platelets, B lymphocytes, T lymphocytes, neutrophils, eosinophils, basophils and macrophages, are derived from pluripotent stem cells which, in the adult, reside mainly in the bone marrow (Lajtha, 1982). The presence of stem cells is inferred by the ability of marrow cells to reconstitute lympho- and myelo-poiesis when transferred into irradiated hosts. However, several months are required before full and lasting reconstitution with donor cells can be unequivocally demonstrated, and for this reason, the shorter term quantitative assay of ‘spleen colony formation’, developed in the 1960’s by Till and McCulloch, is consequently used as an assay for pluripotent haemopoietic cells (McCulloch and Till, 1964). The most primitive spleen colony forming cells (CFU-S) are multipotent, can produce mature cells representative of all the myeloid cell lineages and can undergo extensive self-renewal. Normally, only a minority of CFU-S are undergoing DNA synthesis, but this proportion increases dramatically during recovery from cytoreductive therapy or following bone marrow transplantation (Lajtha, 1982).

The recognition of CFU-S as a minimal-cycling population during steady-state haemopoiesis infers that the massive expansion in cell numbers between the CFU-S and the mature cells occurs via an intermediate population of proliferating cells. These intermediate progenitor cells have been characterised using in vivo assays such as for the erythropoietin responsive cell (ERC) (Bruce and McCulloch, 1964) and in vitro assays, which assess the ability of cells to give rise to colonies of mature haemopoietic cells when immobilised in soft gel media, such as agar or methylcellulose (Metcalf, 1984). The clonal nature of the colonies arising from the progenitor cells has been demonstrated using karyotypic analysis, isoenzyme markers and a number of other techniques (Metcalf, 1984). From these assays it has become apparent that the provision of media containing the appropriate growth stimulator(s) is an essential element in the formation of such colonies and is necessary for the progenitor cells to survive, proliferate and develop. The absence of these ‘colony stimulating factors’ (CSFs) leads to the death of progenitor cells within a very short time scale (8-48 h) (Metcalf and Merchav, 1982).

Using these in vitro assays, a number of distinct, committed, progenitor cell populations have now been identified (see Fig. 1). Such cells are generally limited in their developmental potential to only one or two of the haemopoietic lineages, their proliferation is normally tightly coupled to development and the cells progressively display the antigenic, biochemical and morphological features characteristic of the mature cells of the appropriate lineage. Accompanying this process of development, there is a gradual loss of proliferative potential, resulting in the formation of the postmitotic mature cells (see Fig. 2): in the case of the erythrocyte and the platelet, no inheritable genetic material is retained by the mature cell, while in neutrophils the DNA is retained in a condensed form and the cells are unable to undergo replication. Thus, when cells from these three lineages leave the bone marrow, they possess little or no proliferative potential. Other haemopoietic cells, however, can migrate from the bone marrow and undergo proliferation and development at other specific sites in the body (e.g. pre T cells in the thymus) (Wood, 1982). Similarly, the mast cell and the monocyte can undergo further replication and development in the tissues (Stanley et al. 1983; Tsuji et al. 1990). Although the mechanism of acquisition of a postmitotic phenotype in myeloid cells is not yet understood, an insight into this process is clearly of value in understanding the cellular mechanisms underlying the formation of leukaemic cells from normal haemopoietic tissue. In this respect, identification of the CSFs and other growth factors, and the characterisation of their target cells and modes of action, has implications not only for development of normal haemopoietic cells, but for leukaemic cells also.

Fig. 1.

Haemopoiesis. Cells involved in haemopoiesis can be divided into three main stages: the multipotent cells which have differing degrees of ability to reconstitute the bone marrow of irradiated mice, but nonetheless are multipotent and have the capacity to self-renew; progenitor cells, which are unipotent or bipotent cells committed to a programme of development to form the third stage, which consists of mature cells, displaying a range of grossly different morphologies and functional activities, yet all derived from a common pool of pluripotent stem cells. There are a number of assays which can be employed to identify committed progenitor cells and some of the multipotent cells found in the bone marrow. The CFU-S (Colony Forming Unit-Spleen) assay is an in vivo assay for multipotent, self-renewing stem cells, whilst the high proliferative potential colony forming cell (HPP-CFC Stanley et al. 1986) assay identifies a primitive, multipotential cell using soft agar colony formation as the parameter which is measured (see text). These cells can give rise to the colony forming cell-mix (CFC-Mix) which has a lower capacity for self-renewal but is still multipotent. The multipotent cell then differentiates to give rise to the lineage-restricted, committed progenitor cells. These include the Burst Forming Unit-Erythroid (BFU-E), Colony Forming Unit-Erythroid (CFU-E), Granulocyte Macrophage Colony Forming Cell (GM-CFC), Eosinophil Colony Forming Cell (Eos-CFC), Megakaryocyte-CFC (Meg-CFC) and Basophil-CFC (Bas-CFC).

Fig. 1.

Haemopoiesis. Cells involved in haemopoiesis can be divided into three main stages: the multipotent cells which have differing degrees of ability to reconstitute the bone marrow of irradiated mice, but nonetheless are multipotent and have the capacity to self-renew; progenitor cells, which are unipotent or bipotent cells committed to a programme of development to form the third stage, which consists of mature cells, displaying a range of grossly different morphologies and functional activities, yet all derived from a common pool of pluripotent stem cells. There are a number of assays which can be employed to identify committed progenitor cells and some of the multipotent cells found in the bone marrow. The CFU-S (Colony Forming Unit-Spleen) assay is an in vivo assay for multipotent, self-renewing stem cells, whilst the high proliferative potential colony forming cell (HPP-CFC Stanley et al. 1986) assay identifies a primitive, multipotential cell using soft agar colony formation as the parameter which is measured (see text). These cells can give rise to the colony forming cell-mix (CFC-Mix) which has a lower capacity for self-renewal but is still multipotent. The multipotent cell then differentiates to give rise to the lineage-restricted, committed progenitor cells. These include the Burst Forming Unit-Erythroid (BFU-E), Colony Forming Unit-Erythroid (CFU-E), Granulocyte Macrophage Colony Forming Cell (GM-CFC), Eosinophil Colony Forming Cell (Eos-CFC), Megakaryocyte-CFC (Meg-CFC) and Basophil-CFC (Bas-CFC).

Fig. 2.

The basic structure of haemopoiesis. Multipotent cells can either undergo selfrenewal, or become committed to development. The development of haemopoietic progenitor cells is coupled to proliferation, such that one progenitor cell can give rise to thousands of mature cells. A corollary of this development is the loss of proliferative capacity until postmitotic but functionally active cells (with finite lifetimes) such as neutrophils, platelets, or erythrocytes are formed. The death of these cells requires that they are constantly replaced throughout the lifetime of the organism, thus haemopoiesis is an ongoing process in all healthy animals.

Fig. 2.

The basic structure of haemopoiesis. Multipotent cells can either undergo selfrenewal, or become committed to development. The development of haemopoietic progenitor cells is coupled to proliferation, such that one progenitor cell can give rise to thousands of mature cells. A corollary of this development is the loss of proliferative capacity until postmitotic but functionally active cells (with finite lifetimes) such as neutrophils, platelets, or erythrocytes are formed. The death of these cells requires that they are constantly replaced throughout the lifetime of the organism, thus haemopoiesis is an ongoing process in all healthy animals.

Growth factors which regulate myeloid cell production

Many growth factors responsible for initiating proliferation and development of haemopoietic progenitor cells in vitro have now been molecularly cloned and purified to homogeneity (see Clark and Kamen, 1987; Whetton and Dexter, 1989). The availability of large quantities of highly purified material has facilitated both in vitro and in vivo studies. Both animal and clinical trials have confirmed their physiological significance. The target cell populations which respond to each growth factor, and its effects in vivo, are summarised in the next section.

Interleukin 3 (IL-3)

Of the known haemopoietic growth factors acting on myeloid cells, IL-3 has activity on the greatest range of committed progenitor cells. It can maintain the viability of the CFU-S population in a highly enriched population of cells (precluding the possibility that IL-3 promotes the production of a secondary growth factor from other cell types in the bone marrow which then act on CFU-S) and also initiate DNA synthesis in these cells (Heyworth et al. 1988; Spirak et al. 1985; Schrader and Clark-Lewis, 1982). It can stimulate multipotent cells to develop into colonies containing mature cells from the macrophage, neutrophil, eosinophil, mast cell, megakaryocytic and erythroid lineages (see Whetton and Dexter, 1989). Similarly most of the committed progenitor cells from the myeloid cell lineages will proliferate and develop in response to IL-3. IL-3 can stimulate the development of mature cells from granulocyte macrophage colony forming cells (Cook et al. 1989), mast cell progenitors, eosinophil (Rothenberg et al. 1988), megakaryocyte (Bruno et al. 1988) and erythroid (Sonoda et al. 1988) progenitor cells. Although there is little evidence to suggest that cells committed to T and B lymphocyte development respond directly to IL-3, using haemopoietic reconstitution in irradiated mice as an assay, it has been demonstrated that cells which can give rise to T and B lymphocytes can respond to IL-3 (Clark-Lewis et al. 1985). In some instances there is a requirement for a second growth factor for the complete development of all the progenitor cells: late erythroid progenitor cells require the presence of erythropoietin to form mature erythroid cells (Iscove et al. 1974), and IL-5 is required for eosinophil maturation (Yamaguchi et al. 1988). However IL-3 is generally a powerful mitogen for the early progenitor cells in any given lineage: it is the more mature cells which are relatively less responsive. Thus IL-3 acts at several stages of development of haemopoietic cells, and on cells from several distinct lineages.

Furthermore, some mature cells are responsive to IL-3, although it is notable that erythrocytes and possibly human neutrophils no longer express cell surface receptors for this cytokine. Monocytes are still able to respond to IL-3: treatment with this growth factor not only stimulates survival and maintains the functional activity but also enhances the cytotoxic activity of these cells (Cannistra et al. 1988). This latter activity may be mediated by the potentiation of tumour necrosis factor expression (Cannistra et al. 1987). Similarly, mast cells exhibit an IL-3 potentiated histamine granule release, and connective tissue type mast cells are dependent on IL-3 for their survival and proliferation (Tsuji et al. 1990). Thus, like other myeloid growth factors (see below), IL-3 can potentiate mature cell function in addition to its role in the proliferation and development of haemopoietic progenitor cells.

Interleukin 1 (IL-1)

Some cytokines have no ability to stimulate directly the formation of colonies from normal bone marrow, but can markedly potentiate the proliferation and development of haemopoietic cells in vitro and in vivo in response to growth factors. For example, Bradley, Stanley and co-workers identified preparations of conditioned media from leukaemic cells which, when added to bone marrow cells individually, had no ability to stimulate colony formation; when added together large colonies of mature haemopoietic cells were formed (Stanley et al. 1986). Based on these observations, a biological activity named haemopoietin 1 was purified and characterised from the conditioned medium of leukaemic 5637 cells, on the basis of its ability to stimulate proliferation and differentiation of multipotent cells when combined with macrophage colony stimulating factor (MCSF) (see later). These two agents synergistically promote the development of large colonies containing mature macrophages, but when added alone neither agent stimulates colony formation. Mochizinki et al. (1987) later demonstrated that haemopoietin 1 was identical to Interleukin 1 (IL-1). However, the experiments performed did not supply absolute proof of a direct synergy between these two growth factors acting on a single (multipotent) target cell population. Since Interleukin 1 is known to stimulate the production of a range of cytokines and haemopoietic growth factors from cells found in the bone marrow (Lee et al. 1987), it was possible that IL-1 was acting in a paracrine fashion to stimulate the release of growth factors from such cells which then act in concert with M-CSF to stimulate colony formation (see Bagby, 1989, for a review). Indeed, it has been proposed that such a model is responsible for the actions of IL-1 on multipotent haemopoietic cells (see Ikebuchi et al. 1988a). In more recent work, however, it has been shown that IL-1 can synergise with M-CSF to stimulate the formation of colonies from a cell population highly enriched for CFU-S (Heyworth et al. 1988). This indicates that, at least in some circumstances, IL-1 can directly influence the growth and development of multipotential cells. It is of course possible that IL-1 can act on multipotent cells to stimulate the production of other growth factors, which then act in an intrinsic fashion. With in vivo experiments the situation is, of course, even more complex. But in terms of the effects on haemopoietic cells perhaps the most significant discovery is that IL-1 can decrease the time taken to recover from leucopenia following treatment with cytotoxic drugs such as 5-fluorouracil (Moore et al. 1990).

Interleukin 4 (IL-4)

Originally described as a B cell growth factor (Widmer and Grabstein, 1987), IL-4 has recently been shown to act on a number of other cell types, including myeloid progenitor cells. Early studies suggested that IL-4 could synergise with IL-3 (to stimulate mast cell, but suppress neutrophilic cell, development), erythropoietin (erythroid cell development), G-CSF and M-CSF (neutrophil and macrophage development respectively) (Rennick et al. 1987; Peschel et al. 1987).

Further studies using bone marrow from 5-fluorouracil treated mice (which has few committed progenitor cells, but still contains multipotent, primitive, haemopoietic cells) suggest that IL-4 can promote the differentiation and development of multipotent cells (Kishi et al. 1989). The addition of IL-4 to soft agar cultures of such bone marrow preparations gives a few granulocyte macrophage and blast cell colonies. Further replating of such blast cell colonies resulted in the development of colonies containing cells from several distinct lineages, indicating that IL-4 had supported the proliferation of multipotent cells in the original blast cell colonies.

Interleukin 6 (IL-6)

Interleukin 6 is a pleiotropic cytokine that has the ability to modulate acute phase protein levels, stimulate the production of neutrophils and to stimulate B cell proliferation. Ogawa and co-workers have presented evidence that IL-6 can act on multipotential cells to reduce the G0 period of the cell cycle, and that when the multipotent cells leave this phase of the cycle they then become responsive to IL-3 (Ikebuchi et al. 1988a). In vivo experiments also provided evidence that IL-6 can act on the primitive multipotent haemopoietic cells as well as megakaryocyte precursors; infusion of this growth factor into mice leads to a large increase in the number of CFU-S present in the spleen (Suzuki et al. 1989), and also to an increase in the platelet count. Although it has been suggested that the effects of IL-1 on multipotent cells can be attributed to its ability to stimulate the production of IL-6 from bone marrow stromal cells (Ikebuchi et al. 1988a), our investigations using highly enriched multipotent cells (where no helper cell population is present) suggest that both IL-6 and IL-1 (see above) can act directly on these cells (Heyworth et al. 1988).

The model presented by Sachs and co-workers (1990) may explain this enigma. They suggest that normal myeloid progenitor cells can be stimulated by growth factors such as IL-3, GM-CSF, M-CSF or G-CSF to produce IL-6. This autocrine production of IL-6 then acts to make the progenitor cells differentiate, while the growth factors promote proliferation; in this way proliferation and differentiation are coupled. Of course distinct growth factors may stimulate different levels of production of IL-6, thus the balance between proliferation and development would vary depending on the growth factor employed. Clearly the appropriate techniques are now available to test the validity of this hypothesis using highly enriched (growth factor responsive) progenitor cell populations coupled to an analysis of IL-6 gene expression. However, it should be noted that data do not support the concept of IL-6 acting solely as a differentiation inducer. Using enriched progenitor cells it has been demonstrated that IL-6 can act as a proliferative stimulus. Furthermore there is no evidence that IL-6 induces cellular maturation without accompanying proliferation of such cells (Ikebuchi et al. 1987; Hoang et al. 1988; Suda et al. 1988; Carcacciolo et al. 1989).

Transforming Growth Factor-1 (TGF- β)

TGF-β has a wide variety of effects including stimulation of osteoclast and Schwann cell proliferation, and the inhibition of proliferation of epithelial cells, fibroblasts and endothelial cells (Axelrad, 1990). Recently it has been demonstrated that TGF-β is produced by cells present in the bone marrow (Eaves et al. 1988) and the haemopoietic areas of foetal liver, and evidence from experiments in vitro (using highly enriched marrow progenitor cells) suggest that TGF-/J can directly influence the growth of haemopoietic cells. TGF-β does not stimulate colony formation from progenitor cells, but can inhibit proliferation and development of fluorescence-activated, cell sorted (FACS) purified CFU-S stimulated with IL-3 (Hampson et al. 1989) and also inhibit the proliferation of granulocyte macrophage colony forming cell (GM-CFC) following stimulation with IL-3 and M-CSF. Similarly the addition of TGF-β to colony forming assays for BFU-E or CFU-E leads to inhibited growth (Del Rizzo et al. 1990; Hino et al. 1988; Axelrad et al. 1987). GM-CSF stimulated colony formation is, in some cases, inhibited (Sing et al. 1988) and in others, activated, by TGF-β.

These data firmly indicate a physiological role for TGF-β in the control of haemopoiesis.

Stem Cell Inhibitor (Macrophage Inflammatory Protein 1)

Crude extracts from several tissues, and also from the conditioned medium of cells such as macrophages, have been known for some time to contain an activity which has the ability to inhibit the progression of CFU-S into the cell cycle from Go. This activity is present in haemopoietic tissues where CFU-S can be shown to be quiescent, but is much reduced in regenerating bone marrow, where CFU-S are actively cycling (Graham et al. 1990). Such data suggest that this ‘inhibitor’ may play a role in regulating stem cell proliferation.

Recent attempts to isolate, purify and obtain a partial sequence for this stem cell inhibitor by Pragnell and co-workers have met with success, and it has now been shown that the CFU-S inhibitor is probably the same molecular entity as Macrophage Inflammatory Protein la (MIP-lo) (Graham et al. 1990). At present the range of target cells on which this cytokine acts have not been established, although Broxmeyer and co-workers have shown a significant enhancement of granulocyte macrophage colony formation in the presence of MlP-ln, using highly enriched GM-CFC (Broxmeyer et al. 1989). It will now be of some interest to determine the in vivo effects of this novel cytokine, particularly on stem cell proliferation.

The negative regulator of CFU-S entry into DNA synthesis, characterised and purified by Guigon, Frindel and co-workers, has already been shown to be effective in vivo as a proliferation inhibitor (Lenfant et al. 1989). Surprisingly, this molecule is a simple, acetylated tetrapeptide, although this may be a fragment of a larger molecule found in vivo (Axelrad, 1990; Lenfant et al. 1989). Undoubtedly, though, this molecule, MIP-lo, TGF-β and other inhibitors of stem cell proliferation such as inhibin (Axelrad, 1990), will be the focus of a great deal of attention because of their possible use in the treatment of patients who may risk bone marrow failure as a result of chemotherapy, and also because the failure of stem cells to respond to such inhibitors may be one of the underlying causes of malignant disease (see Dexter and White, 1990).

Granulocyte Macrophage Colony Stimulating Factor

In in vitro assays, murine GM-CSF can stimulate the development of colonies containing predominantly neutrophils and/or macrophages, and also eosinophils. Higher concentrations of the growth factor can also stimulate the development of megakaryocytic cells, and stimulate the proliferation and development of early erythroid progenitor cells (BFU-E) (see Whetton and Dexter, 1989; Metcalf, 1984). There is also evidence that some multipotent cells can respond to GM-CSF. This ability to act on the immature myeloid progenitor cell does not, however, define the full range of biological activity of this growth factor.

The mature myeloid cells also exhibit a profound response to GM-CSF: peripheral blood monocytes and tissue-based macrophages proliferate in response to GM-CSF and also show an enhanced cytotoxic activity (Grabstein et al. 1986; Reed et al. 1987). Monocytes can also be stimulated by GM-CSF to release prostaglandin E, arachidonic acid, IL-1, tumour necrosis factor and other cytokines such as M-CSF (Di Persio et al. 1989; Arnaout et al. 1986; Cannistra et al. 1987). In addition to these effects, GM-CSF can prime the functional activity of mature, circulating neutrophils; both the phagocytic and cytotoxic activity of these cells is markedly enhanced by preincubation with this growth factor (Golde et al. 1990; Fleischmann et al. 1986). Primed neutrophils are more effective in the phagocytosis and killing of yeast, bacteria and opsonised tumour cells (Fleischmann, 1986; Villallta and Kierszenbaum, 1986). These effects are in part achieved by enhancing the superoxide anion production of the neutrophils, as well as enhancing the rate of phagocytosis.

Another facet of the biological activity of GM-CSF, which is perhaps just as physiologically relevant, is its effect on neutrophil locomotion and adhesion. GM-CSF is a chemotactic factor for phagocytic cells, and its production can be stimulated in macrophages (e.g. in response to lipopolysaccharide or interferon-y) (Hamilton and Adams, 1987; Thorens et al. 1987; Piacibells et al. 1985; Brussollino et al. 1989) and endothelial cells (in response to agents such as lipopolysaccharide and tumour necrosis factor). This may be of significance in the movement of neutrophils to areas of infection. Recent reports also suggest that the adhesion of neutrophils to endothelium is enhanced following treatment of the cells with GM-CSF. These data indicate a major role for GM-CSF in response to infectious agents (Gamble et al. 1989).

Haemopoietic cells are not the only cell types which respond to GM-CSF. It can also stimulate the proliferation and chemotaxis of endothelial cells. Thus at sites of injury, GM-CSF can potentially attract both phagocytic cells and endothelial cells. This suggests that GM-CSF is a pleiotropic cytokine which can not only stimulate the development of myeloid progenitor cells, but also mediate the inflammatory response associated with infection and the process of wound healing.

Granulocyte Colony Stimulating Factor (G-CSF)

Originally, Granulocyte Colony Stimulating Factor was purified on the basis of its ability to induce the differentiation of the WEHI-3B myelomonocytic leukaemia cell line, and to stimulate the development of mature neutrophils from normal bone marrow progenitor cells in colony forming assays (Burgess and Metcalf, 1980; Platzer et al. 1983). Recent data has extended the range of activity of this growth factor to a number of other cells, including endothelial cells, mature, postmitotic neutrophils (see below) and multipotent haemopoietic stem cells. Like IL-6, the serum levels of this protein rise markedly in mice after the injection of endotoxin, suggesting a role for these agents in the response to infections.

There is evidence that, like IL-6, G-CSF can shorten the Go period of the dormant multipotent cell in the bone marrow (Ikebuchi et al. 1988b). In the presence of G-CSF, some progenitor cells present in bone marrow from 5-fluorouracil treated mice form blast cell colonies, but when IL-3 is also added there is a synergistic interaction between these factors, leading to the development of colonies containing mature cells from several lineages. These synergistic effects are also seen using highly enriched populations of multipotent cells, inferring that the effects are not mediated through the paracrine production of secondary growth factors (see above) (Heyworth et al. 1988).

In addition to the effects of G-CSF on multipotent cells, and as a stimulus for the development of neutrophilic cells, there are a number of other biological effects of this growth factor. In many respects, these are similar to the effects seen with GM-CSF. For example, G-CSF can increase the lifetime of circulating neutrophils, and enhance their antibody-dependent cellular cytotoxicity (Begley et al. 1986; Vadas et al. 1983; Tsuchiya et al. 1986), although it is of interest to note that this effect is additive with that of GM-CSF, suggesting that they do not share a common priming mechanism (Vadas et al. 1983, 1985). However, both GM-CSF and G-CSF enhance the production of reactive oxygen intermediates by neutrophils in response to the chemotactic peptide formyl-methionyl-leucylphenylalanine, and can also stimulate endothelial cell proliferation and chemotaxis. The dramatic elevation in GM-CSF and G-CSF levels in bacterially-infected mice (Burgess and Nicola, 1983), and the ability of G-CSF treatment to reduce the incidence of infection in patients (Bronchud and Dexter, 1989), suggest that G-CSF has a role in host defence mechanisms via its effects on myeloid cell production and functional activity.

Interleukin 5

Human Interleukin 5 (IL-5) can promote the development of eosinophilic progenitor cells, whilst its murine counterpart (which is 70% homologous) can also promote the proliferation of B cell precursors. Evidence from murine model systems suggests that IL-5 has a role in vivo, in the systemic response to parasitic infections (Strath and Sanderson, 1986). Serum levels of this cytokine are markedly elevated in mice infected with parasites, and this is followed by an increase in eosinophil production from the bone marrow, and eosinophil numbers in the peripheral blood (Strath and Sanderson, 1986). Infusion of exogenous IL-5 has a similar effect (Sanderson et al. 1986).

Furthermore, the survival of eosinophils, and their cytotoxic activity, is markedly potentiated by IL-5, which is also a chemotactic factor for these cells (Yamaguchi et al. 1988b). Thus, like G-CSF and GM-CSF, IL-5 not only facilitates the proliferation and development of eosinophils but also the capacity of the mature cells to perform their specific functions. However, unlike the effects of GM-CSF or G-CSF on phagocytic cells, IL-5 does not prime eosinophils to display enhanced levels of superoxide production (see above) from eosinophils, it directly activates superoxide production by eosinophils (Yamaguchi et al. 1988a).

Macrophage Colony Stimulating Factor (M-CSF)

In soft agar cultures of normal murine bone marrow, M-CSF (or Colony-Stimulating Factor 1) stimulates the formation of predominantly macrophage colonies, derived from granulocyte macrophage colony forming cells. The response of multipotent cells to M-CSF is limited unless a second cytokine such as IL-1 is present. Interestingly, human M-CSF, although a potent growth factor for murine GM-CFC, is relatively poor in its ability to promote the development of colonies from human bone marrow; the reason for this is unknown.

M-CSF does, however, support the survival and proliferation of tissue based macrophages and monocytes. M-CSF can also affect the functional activity of mature macrophages. For example, the production of prostaglandins, plasminogen activator, interferons, tumor necrosis factor, and GM-CSF have all been shown to increase in M-CSF treated macrophages. Similarly M-CSF promotes the cytotoxic and phagocytic capacities of these cells, as well as priming them to produce greater quantities of reactive oxygen intermediates (see above).

Model systems to study haemopoietic stem and progenitor cell development

It has been suggested that the effects of the different growth factors on haemopoietic stem cells resemble those of the so-called competence and progression factors in fibroblastic cell proliferation (Rozengurt, 1986). Agents such as IL-6, G-CSF and IL-1 can stimulate the transition of the stem cells from a Go cell cycle state into Gi, and as such can be thought of as ‘competence factors’. These cells are then able to respond to a second set of factors (such as IL-3 and also GM-CSF) which can stimulate the progression of these cells into S phase, and development into committed progenitor cells. While the growth factors mentioned above undoubtedly have the ability to influence the proliferation and development of multipotent haemopoietic cells, there is little information on the biochemical mechanisms mediated by these factors and leading to regulation of cell cycle progression, stem cell differentiation, lineage restriction and the development of the postmitotic phenotype.

The most appropriate population for such studies are stem cells or committed progenitor cells prepared from in vivo sources. However, there are difficulties in the preparation of such populations, in that the bone marrow contains a heterogenous population of which immature blood cells represent a minor proportion. The use of drugs such as cyclophosphamide or thiamphenicol can increase the proportion or numbers of the progenitor cells required. This, and recent advances in cell purification procedures (such as centrifugal elutriation, or cell surface antigen-based selection techniques) have produced relatively pure populations of stem and committed progenitor cells (Lord and Spooncer, 1986; Williams et al. 1987). Unfortunately, these procedures give a relatively low yield of the required cells, often insufficient for molecular, biological or biochemical analysis. However, given the very low number of cells required for colony forming assays, sufficient cells can usually be obtained to determine if there is a direct (as opposed to a paracrine) effect of mitogenic agents on enriched cells (see above). Enriched populations of CFU-S, GM-CFC, CFU-E or BFU-E have also been employed to investigate the types of cytokines that can act directly on immature haemopoietic cells (Heyworth et al. 1990; Williams et al. 1987; Cook et al. 1989; Mitler et al. 1989), and the possible biochemical events they elicit within the cell (Mitler et al. 1989; Cook et al. 1989; Imagawa et al. 1989). Although the results obtained give an insight into the nature of the target cell populations for the various cytokines, the limited numbers of cells that can be prepared precludes investigation of the molecular mechanisms regulating stem cell commitment, lineage restriction, and development to mature cells. For this reason, many of these events are presently being studied using a variety of cell lines.

Cell lines for the study of myeloid growth factor stimulated development

Perhaps the most frequently employed cell line for biochemical studies on the maturation of myeloid cells is the human promyelocytic leukaemic cell line, HL-60. In response to a wide variety of extracellular stimuli (such as retinoic acid phorbol esters and dimethylsulphoxide), HL-60 cells will develop into granulo-cyte-, macrophage- and also eosinophil-like cells, although differences do exist between these cells and normal granulocytes and macrophages (Collins, 1987). Although the developmental stimuli which have been used in the past to induce HL-60 cells have no physiological role in haemopoiesis in vivo, there is evidence that GM-CSF can also induce differentiation (Metcalf, 1983; Begley et al. 1987). Another important distinction between HL-60 cells and normal myeloid precursors is that HL-60 cells will grow in simple defined medium in the absence of any haemopoietic growth factor (Collins, 1987), whereas normal progenitor cells rapidly die in such conditions. In this respect, HL-60 cells can be distinguished from the majority of primary myeloid leukaemia cells, which also require growth factors for survival and proliferation.

These marked distinctions between normal progenitor, primary myeloid leukaemia and HL-60 cells, suggest that the latter may not be an appropriate model to study many aspects of myeloid cell development. Similar criticism applies to the human leukaemic cell line K562, which can develop along several distinct lineages in response to a variety of non-physiological stimuli (Ohlsson-Wilhelm et al. 1987; Leary et al. 1987; Nishimura, 1988).

There are other cell lines, however, which can differentiate to mature cells in response to specific haemopoietic growth factors. For example, the M1 and WEHI-3B and 32DC13 cell lines will undergo differentiation to mature cells in response to G-CSF (Valtieri et al. 1987). Also the IL-3 dependent, multipotential, LyD9 (Kinachi et al. 1989) and FDCP-Mix (Spooncer et al. 1986) cell lines can be stimulated by a number of distinct cytokines to develop into the appropriate mature cell type.

We have employed the FDCP-Mix cells to study the balance between self-renewal and differentiation in haemopoietic cells. These events are apparently regulated not only by the type of growth factors in which the cells are cultured, but also by the concentrations of these factors. When combined with a low concentration of IL-3, the cells respond to GM-CSF, M-CSF, G-CSF or erythropoietin by developing into mature postmitotic cells. The type(s) of mature cells produced are governed by which growth factor(s) are added (e.g. erythropoietin stimulates erythroid development). At a high concentration of IL-3, the cells proliferate and self-renew; they do not differentiate, irrespective of the presence of other growth factors. In the absence of IL-3 the cells show little or no response to other growth factors.

Initially we have investigated the mechanisms regulating the development of FDCP-Mix cells to neutrophils. In the presence of high concentrations of IL-3, FDCP-Mix (A4) cells undergo self-renewal, irrespective of the presence of other haemopoietic growth factors such as GM-CSF or G-CSF. At a low concentration of IL-3, in the presence of GM-CSF plus G-CSF, FDCP-Mix cells become committed to development and produce predominantly neutrophils. The concentration of IL-3 dictates whether the cells self-renew or differentiate, that is to say there is an antagonism between IL-3 and stimulators of neutrophilic development. IL-3 has an ability to promote self-renewal, but how does it antagonise the developmental response given to multipotent A4 cells by growth factors such as GM-CSF?

The FDCP-Mix cell lines offer the opportunity to examine the biochemical events elicited by haemopoietic growth factors leading to differentiation and development. Using these cells and the preparations of highly enriched, normal, bone marrow-derived progenitor cells such as GM-CFC (Cook et al. 1989) and CFU-S (Heyworth et al. 1988) it will now be possible to assess the molecular mechanisms which regulate the survival, proliferation, differentiation and development of apparently normal haemopoietic stem and progenitor cells. This combination of cellular biological and biochemical approaches will not only allow the mechanisms regulating normal haemopoietic cell development to be identified, but also reveal how these mechanisms are restricted during leukaemic transformation.

Work in the authors’ laboratories is funded by the Cancer Research Campaign (T.M.D.) and the Leukaemia Research Fund (A.D.W.).

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